22 



Mr. W. H. M. Christie on the 



[Mar. 1, 



tially corrected in the direct-vision prism by increasing the angle of the 

 crown. The prism would, however, no longer be direct-vision, even for 

 the ultra-violet, the deviation being negative and considerable in amount 

 for the visible rays. 



So far the case has been considered in which the pencils for different 

 parts of the spectrum all pass in the same direction through the prism 

 (parallel to the base), and the collimator and telescope are moved ; but it 

 is also possible to keep the collimator and telescope fixed and to turn the 

 prism merely. In the former plan, which is that usually adopted, the 

 limit to the angle of the prism is fixed by the condition that the angle of 

 incidence or emergence for the violet at the oblique face must not be too 

 large, or, in other words, that the angle of refraction must not be very 

 near the critical angle for the violet, which is smaller than that for the 

 red ; and consequently great dispersion is accompanied with correspond- 

 ing irrationality, except in the case of the direct-vision prism. If, how- 

 ever, the deviation be kept the same for all parts of the spectrum — which 

 can be done by turning the prism (provided it be not isosceles) — this 

 objection is got rid of, and at the same time the construction of the 

 spectroscope is made much more simple. The refracting angle of a half- 

 prism, whether simple or compound, can thus be increased, so as to give 

 the most suitable dispersive power for the middle of the dispersion spec- 

 trum (about F) when the pencil is incident perpendicularly on the first 

 face. By turning the prism successively in one direction or the other, 

 so as to increase the angle of refraction for the red or diminish it for the 

 violet, the different parts of the spectrum can be made to emerge suc- 

 cessively in the same direction, and the dispersive and magnifying-powers, 

 as well as the loss of light, will be sensibly constant throughout the spec- 

 trum, the dispersion at the first face being extremely small. Of course, 

 if the whole spectrum be included in the field of view there will be the 

 usual inequality of scale ; but such an extent of field can only be used 

 for viewing a spectrum with small dispersion; and for purposes of mea- 

 surement, the part examined must be brought to the middle of the field. 

 It may be remarked that the dispersion may be reduced to about one half 

 by turning the half-prism till the angles of incidence and emergence 

 become equal, when it acts as an isosceles prism at minimum deviation. 

 This property is useful for the examination of faint objects, the decrease 

 in dispersion being further accompanied by an increase in the transmitted 

 light. The spectrum, however, would in this case be thrown some 3° 

 out of the centre of the field, and a motion either of the viewing-tele- 

 scope or of the eyepiece would be necessary. 



In fig. 3 is shown the course of a pencil of red rays (full Hues) and of 

 violet rays (dotted lines) within a direct-vision half -prism as it is turned 

 about a centre in the manner just described. The rays, whether red or 

 violet, emerge parallel to their original direction, the diagram represent- 

 ing the course relative to the prism as it is turned. 



